In recent years, for avoiding deterioration of signals caused by repeated copies, digitization of magnetic recording media has proceeded. Further, the recording density thereof has also been desired to be increased, because of an increase in the amount of recording data.
As magnetic powders, therefore, finer magnetic metal powders having high saturation magnetic flux density have been used. However, when such magnetic metal powders are used, the dispersibility of the magnetic metal powders greatly affects the characteristics of the media.
That is to say, the poor dispersibility of the magnetic metal powders and other inorganic pigments deteriorates the surface properties of the media and the magnetic characteristics, which causes deterioration of the electromagnetic characteristics (output) and the durability.
As measures to solve such a problem, it has been studied that the dispersibility of the powders is improved by the use of binders having polar groups high in adsorptivity to the magnetic metal powders.
Specific examples thereof include the use of binders comprising polymers having --COOX, --SO.sub.3 X, --OSO.sub.2 X, --PO.sub.3 X.sub.2 or --OPO.sub.3 X.sub.2 as the polar groups [wherein X represents --NR.sub.1 R.sub.2 R.sub.3 R.sub.4 (R.sub.1, R.sub.2, R.sub.3 and R.sub.4 each represents a hydrogen atom, or an alkyl, alkenyl, substituted alkyl or substituted alkenyl group having 1 to 12 carbon atoms)] (Japanese Unexamined Patent Publication No. 63-261523), the use of binders composed of resin components having --COOM, --SO.sub.3 M, --OSO.sub.3 M or --PO(OM').sub.2 as the polar groups [wherein M represents a hydrogen atom or an alkali metal atom, and M' represents a hydrogen atom, an alkali metal atom or a lower hydrocarbon group] (Japanese Examined Patent Publication No. 6-10860, Japanese Unexamined Patent Publication No. 8-165372, Japanese Unexamined Patent Publication No. 8-77460 and Japanese Unexamined Patent Publication No. 9-265623), and the use of binders containing polyurethane resins in which negative functional groups form internal salts (Japanese Unexamined Patent Publication No. 2-101624).
However, with the recent increase in capacity of digital recording media, head gaps become narrow to bring about the liability of heads to clog, and the number of repetitions of reading and writing is also increased. Accordingly, higher levels of the dispersibility and durability have been desired particularly in disk media, so that it has also become necessary to enhance the dispersibility of carbon black and abrasives which are additives used together with the magnetic powders.
When the binders having the polar groups described in Japanese Unexamined Patent Publication No. 63-261523 and the like described above are used, the dispersibility of the magnetic metal powders is improved, but a balance between the paint stability or the paint viscosity and the yield value thereof is insufficient. They are therefore not satisfactory in this respect.
As a measure for preventing the clogging of heads, addition of Cr.sub.2 O.sub.3 has hitherto been known, and as a means for enhancing the durability, it has formerly been known that large-sized carbon is added to reduce the friction of coated films. However, the polar groups such as --COOM, --SO.sub.3 M, --OSO.sub.3 M (wherein M is K, Na or H) and metal salts of sulfamic acid, shown in Japanese Unexamined Patent Publication No. 63-261523, Japanese Examined Patent Publication No. 6-10860, Japanese Unexamined Patent Publication No. 8-165372 and Japanese Unexamined Patent Publication No. 9-265623 described above, improve the dispersion of the magnetic metal powders but the total dispersion became insufficient, because the material characteristics of carbon or Cr.sub.2 O.sub.3 were largely different from those of the magnetic metal powders. On the other hand, the use of amine polar groups which have previously been known to provide good dispersion of carbon or Cr.sub.2 O.sub.3 resulted in the difficulty in dispersing the magnetic metal powders. Further, even if resins having acid polar groups excellent in the dispersion of the magnetic metal powders, such as --COOM, --SO.sub.3 M, --OSO.sub.3 M (wherein M is K, Na or H) and metal salts of sulfamic acid, shown in Japanese Unexamined Patent Publication No. 63-261523, Japanese Examined Patent Publication No. 6-10860, Japanese Unexamined Patent Publication No. 8-165372 and Japanese Unexamined Patent Publication No. 9-265623, and resins having amine polar groups are used as mixtures, the synergistic effect can not be obtained.
That is to say, the media prepared by such resin mixture systems have the problem that carbon and Cr.sub.2 O.sub.3 are liable to drop out of coated films to cause the clogging of heads and a reduction in durability, because the resins having acid polar groups are incorporated therein. As to the magnetic metal powders, the inconvenience also has occurred that the dispersibility becomes insufficient to deteriorate the surface properties, resulting in difficulty in obtaining high output, because the resins having the amine polar groups are incorporated therein. Further, coated surfaces formed are significantly affected by the dispersion of paints, and paints insufficiently dispersed have the inconvenience that fine coating streaks are developed, when the paints are applied. In particular, this phenomenon has been significant in coating by the wet-on-wet process.
Media having the coating streaks have large spacing loss between heads and the media, so that the output is low and the durability is also poor, which has been a great factor for deteriorating the quality of the magnetic recording media.